1. Field of the Invention
The present invention relates to a method for developing an electrostatic latent image formed on the surface of a photosensitive body, such as a photoconductive drum used for electrophothographic devices and laser printers with one-component developers.
2. Description of the Related Art
It is known to use one-component developers and two-component developers for developing latent electrostatic images.
A two-component developer includes toner particles which actually form a visible image from the latent image and carrier particles for properly charging the toner. In such two-component developers, the mixing ratio of toner to carrier, i.e., the toner concentration, must be kept constant. However, it is difficult to maintain toner concentration constant. A one-component developer has an advantage in that concentration control is not necessary, since toner is the only component of the developer.
There have been proposed developing methods employing the one-component type toner containing no magnetic powder, which is used in the conventional two-component type developing method, but offering a high specific resistance.
The aforesaid methods include those disclosed in U.S. Pat. Nos. 2,895,847 and 3,152,012 and Japanese Laid open Application Nos. 9475/66, 2877/70 and 3624/79, using the touch-down, impression or jumping method.
Use of the toner employed in the two-component type developing method for the one-component developing method still poses the following problems:
In the first place, the amount of the triboelectric charge generated is insufficient when the above method is used. In the one-component developing method generally, the toner relative to a toner conveyer must be charged efficientry for an extremely short time and obtain a charge amount (e.g., between about -0.5 -15 .mu.C/g when a selenium photosensitive drum is used) sufficient to convert an electrostatic latent image formed on a photosensitive drum or dielectric into a visible image in a non-contact state. However, the problem is that the toner cannot be charged enough to carry out the aforesaid image visualization by friction between the toner used in the conventional two-component type developing method and the toner conveyer. In other words, although time is consumed to charge the toner and the carrier to the extent that the charge amount is sufficient to implement image visualization in the conventional two-component developing method, the triboelectric charge time consumed to charge the toner and the toner conveyer by friction in the one-component developing method is too short to provide the charge amount necessary for the image visualization.
Secondly, the surface of a toner conveyer must be covered uniformaly with an extremely thin toner layer, but such a thin layer is impossible to form with the toner employed in the two-component type developing method.
Thirdly, since the major part of the toner is composed of resin, a great percentage of resin exists on the surface of the toner. A pigment in general is negatively charged and, particularly in the case of carbon black, it is negatively charged. The resin is negatively charged by the friction with the elastic blade. This causes an opposite polarity to be produced by the charge generated on the surface of the toner particle between other toner particles; between the toner and the toner conveyer; and between the toner and the elastic blade.
Fourthly, there is the problem that when the toner is transferred from the toner conveyer onto the electrostatic latent image, toner is caused to reciprocate between the toner conveyer and the photosensitive drum. Although a visible image will be formed, if the specific gravity of the toner is low, unnecessary toner will be scattered on the photosensitive drum and will lead to fogging.
Consequently, problems such as development of fog and the scattering of toner may occur. The aforesaid problems frequently occur, particularly when a large volume of documents are copied. Therefore, the conventional one-component type toner is impractical for use in a copying machine.